Vertical crankshaft engines are preferred in some applications because of the possibility of simplifying the structure of the power train. Typical applications of vertical crankshaft engines include lawn mowers, grass trimmers, and marine outboard engines. Such applications typically require the engine to be as compact and light-weight as possible. However, when the engine is constructed as a four-stroke engine, it is highly preferable to lubricate the valve mechanism and the cylinder surface, and this necessitates the provision of an oil sump at the bottom end of the engine as well as other complex oil passages and oil seals.
As one such lubrication system, it was proposed in Japanese patent laid-open publication 61-175209 to utilize the negative pressure produced in the crankcase as the piston moves away from it to introduce lubricating oil into the crankcase and the positive pressure produced in the crankcase as the piston moves towards it to push lubricating oil back to a lubricating oil tank through a valve mechanism. However, according to this dry sump lubrication system utilizing crankcase pressure caused by the movement of the piston, it is difficult to achieve a uniformity of oil distribution, and a time lag in supplying oil to various parts of the engine is inevitable to a certain extent.
In a vertical crankshaft engine, its camshaft is also vertical and is driven in synchronism with the crankshaft by a gear train (as disclosed in Japanese UM publication No. 50-42241) or a chain and sprockets (as disclosed in Japanese patent laid open publication No. 60-113031). However, these power transmission systems produce relatively large noises during their operation. Further, they require a relatively large amount of lubricating oil for their lubrication, and it hampers compact and light weight design of such general-purpose engines.
It is possible to use a so-called cogged belt as a timing belt. However, according to conventional arrangement, the timing belt is passed around the pulleys provided at upper ends of the crankshaft and the camshaft, and not only the timing belt is placed in a high temperature condition during the operation of the engine but also air cooling of the engine itself is seriously obstructed by the presence of the timing belt in an upper part of the engine.